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Zinc chromate: structure, properties, obtaining, uses

The term ‘zinc chromate’ commercially serves to designate three compounds with different molecular structure: (a) the zinc chromate itself ZnCrO 4 , (b) the basic zinc chromate ZnCrO 4 • 4Zn (OH) 2 , and (c ) basic zinc potassium chromate 3ZnCrO 4 • Zn (OH) 2 • K 2 CrO 4 • 2H 2 O.

Structure of zinc chromate. Author: Marilú Stea.

It is mainly used in paints or primers that protect metals from corrosion. For this, it is mixed with paints, varnishes and polymers that are then applied to the surface of metals.

It is also used in decorative and protective finishes achieved with other chromates and acids coating various objects such as tools. It also serves to retain the electrical conductivity of metal parts.

It is a material that causes cancer and this is because chromate has chromium in the +6 oxidation state.

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Structure

Zinc chromate is an ionic compound formed by the zinc cation Zn 2+ and the chromate anion CrO 2- . The latter is made up of chromium with valence +6 (hexavalent chromium, Cr 6+ ) and four oxygen atoms with oxidation state -2.

The Zn 2+ ion has the following electronic structure:

1s 2 , 2s 2 2p 6 , 3s 2 3p 6 3d 10 .

For its part, hexavalent chromium has the following conformation in its electronic orbitals:

1s 2 , 2s 2 2p 6 , 3s 2 3p 6 .

Both structures are very stable since the orbitals are complete.

Nomenclature

  • Zinc chromate
  • Chromic Acid Zinc Salt
  • Zinc yellow (although this term also refers to other compounds that contain ZnCrO 4 ).

Properties

Physical state

Lemon yellow or yellow crystalline solid. Crystals in the form of prisms.

Molecular weight

181.4 g / mol

Melting point

316 ºC

Density

3.40 g / cm 3

Solubility

Weakly soluble in water: 3.08 g / 100 g of H 2 O. It dissolves easily in acids and in liquid ammonia. Insoluble in acetone.

pH

According to some sources, its aqueous solutions are acidic.

Chemical properties

It is a strongly oxidizing compound, so it can react with reducing agents, generating heat. Among the substances with which it can react are organic ones, such as cyanides, esters and thiocyanates. It can also attack some metals.

Chromate-formed species

Above pH 6 the chromate ion CrO 2- (yellow in color) is present; between pH 2 and pH 6, the ion HCrO  and the dichromate Cr 2 O 2- (orange-red in color) are in equilibrium ; at pH less than 1 the main species is H 2 CrO 4 .

When zinc (II) cation is added to these aqueous solutions, ZnCrO 4 precipitates .

The balances are as follows:

HCrO  ⇔ CrO 2- + H +

2 CrO 4 ⇔ HCrO  + H +

Cr 2 O 2- + H 2 O ⇔ 2 HCrO 

In basic medium the following occurs:

Cr 2 O 2- + OH  ⇔ HCrO  + CrO 2-

HCrO  + OH  ⇔ CrO 2- + H 2 O

ZnCrO 4 does not react rapidly with air or water.

Obtaining

It can be produced by reacting an aqueous zinc oxide or hydroxide sludge with a dissolved chromate salt and then neutralizing.

Industrially, the Cronak process is used, in which zinc metal is immersed in a solution of sodium dichromate (Na 2 Cr 2 O 7 ) and sulfuric acid (H 2 SO 4 ).

It can also be prepared by precipitating it from solutions in which there are dissolved zinc and chromate salts:

2 CrO 4 + ZnSO 4 → ZnCrO 4 ↓ + K 2 SO 4

Applications

In metal protection

In the metallurgical industry it is mainly used in base paints (preparatory paint or initial coating) applied to metals, to which it provides resistance against corrosion.

It is used as a pigment in paints and varnishes, being inserted into the matrix of an organic polymer.

This type of paint is applied to pipelines, oil tankers , steel structures such as bridges, power transmission towers, and automobile parts to inhibit corrosion.

The steel structures of the bridges are painted with a zinc chromate base before the final painting to protect them from corrosion. Author: オ ギ ク ボ マ ン サ ク. Source: Pixabay.

Passivation

It is also found protecting zinc-coated metal components that have been passivated using alkali metal chromates. Passivation consists of the loss of chemical reactivity under certain environmental conditions.

These coatings also serve as decorative finishes and to retain electrical conductivity. They are commonly applied to everyday items such as tools and can be recognized by their yellow color.

Some tools are coated with zinc chromate. Author: Duk. Source: Wikimedia Commons.

How does it work

Researchers found that zinc chromate’s protection against metal corrosion could be due to its inhibition of fungal growth. In this way it prevents the deterioration of the anticorrosive paint coating.

Other studies indicate that the anticorrosive effect could be due to the fact that the compound accelerates the formation of protective oxides on metals.

Anticorrosive zinc chromate primer for protection of metal surfaces. 水水 / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

In reaction catalysis

This compound has been used as a catalyst in various chemical reactions, such as the hydrogenation of carbon monoxide (CO) to obtain methanol (CH 3 OH).

Esters can be converted to primary alcohols by hydrogenation, using this compound to speed up the reaction.

According to some researchers, its catalytic action is due to the fact that the solid does not present a stoichiometric structure, that is, it deviates from its formula ZnCrO 4 and is rather:

Zn 1-x Cr 2-x O 4

Which implies that there are defects in the structure that energetically favor catalysis.

Other apps

It is found in some oily colorants, it is suitable for printing, it is a surface treatment agent, it is applied in floor coverings and it is a reagent in chemical laboratories.

Discontinued uses

Since the 1940s, a derivative of ZnCrO 4 , zinc copper chromate, has been used as a foliar fungicide for potato plants .

Potato plants. Author: Dirk (Beeki®) Schumacher. Source: Pixabay.

This use has already been abandoned due to the toxicity and harmful effects of the compound.

In artistic paintings from the 19th century, the presence of a complex zinc chromate salt, 4ZnCrO 4 • K 2 O • 3H 2 O (hydrated zinc and potassium chromate), has been found, which is a yellow pigment called Lemon Yellow.

Risks

Although it is not combustible, when heated it emits toxic gases. May explode if in contact with reducing agents or organic materials.

The dust irritates the eyes and the skin producing an allergic reaction. Inhalation causes nose and throat irritation. It affects the lungs, causes shortness of breath, bronchitis, pneumonia, and asthma.

Its ingestion affects the digestive tract, liver, kidneys , central nervous system , produces a circulatory collapse and damages the immune system.

Cancer generator

It is a confirmed carcinogen, increases the risk of lung and nasal cavity cancer. It is toxic to cells (cytotoxic) and also damages chromosomes (genotoxic).

Zinc chromate causes lung and respiratory cancer. Author: OpenClipart-Vectors. Source: Pixabay.

The toxicity and carcinogenicity of this compound has been determined to be caused mainly by the action of chromium in the +6 oxidation state. However, the presence of zinc gives the product insolubility and this also influences the damage it produces.

Effects on the environment

It is very toxic to animals and aquatic life, causing harmful effects that last over time. This chemical can bioaccumulate throughout the food chain.

For all these reasons the processes that involve chromates (hexavalent chromium) are being regulated by world health organizations and replaced by alternative techniques without this ion.

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